Non-emissive grids



United States Patent NON-EMISSIVE GRIDS Martin L. Perl, New York, N. Y., assignor to General Electric Company, a corporation of New York Application August 3, 1951, Serial No. 240,098

2 Claims. (Cl. 148-32) My invention relates to improved non-emissive grids particularly suitable for use with electric discharge devices employing thoriated-tungsten cathodes.

Electron emission from the grid electrode of electric discharge devices has long been a problem in the design of these devices. The emitted electrons may be due to contamination of the grid by material evaporated or sputtered from the cathode and the electrons may also result from primary and secondary emission from the grid material itself. Many attempts have been made to solve this problem and many different materials have been employed. Where large structures are involved and small openings in the grid are not required, graphite control members have been used with considerable success. Molybdenum and platinum-clad molybdenum have been used with considerable success where the temperatures of the grid have not exceeded 1200 C., for example.

At the demand for higher power, higher frequency tubes has increased, it has become almost essential to use thoriated-tungsten or thoria coated cathodes to secure adequate emission and also to operate the grids at relatively high temperatures. This had imposed a combination of conditions upon the grid with respect to the grid emission problem which has not been solved in a successful manner. The present invention relates to improved grids which will operate successfully in tubes employing a thoriated-tungsten cathode at temperatures as high as 1600 C. The present invention also provides a grid which does not result in poisoning of the cathode with a resulting reduction in emission of the cathode during life of the tube.

It is accordingly an important object of my invention to provide a new and improved control grid having superior non-emitting characteristics.

It is a further object of my invention to provide an improved non-emitting grid particularly adapted for use in electric discharge devices employing thoriated-tungsten cathodes.

Further objects and advantages of my invention will become apparent as the following description proceeds, reference being had to the accompanying drawing and its scope will be pointed out in the appended claims. In the drawing, Fig. 1 illustrates a grid wire embodying my invention, Fig. 2 illustrates a grid assembly embodying my invention, Fig. 3 illustrates schematically apparatus for coating grid wire in accordance with my invention and Fig. 4 shows similar apparatus for coating preformed grids.

Referring now to Fig. l of the drawing, I have shown in section a core wire 1 suitable for a grid conductor having an exterior coating of graphitic carbon and an intermediate layer of a carbide or sub-carbide of the metal of the wire. While the refractory metals including molybdenum, tantalum, tungsten and columbium may be employed for the grid wire, I have found that tantalum is easier to process and should be considered the preferred material.

A suitable process for producing a non-emissive grid conductor of the type described in the preceding paragraph 2,821 ,49 b Patented Jan. 28, 1958 will now be described. In general, the grid or grid wire is heated to an elevated temperature, such as 1400 to 2000 C. and subjected to a saturated vapor of a hydrocarbon or hydrocarbon derivative that vaporizes without decomposition. The vapor is carried by a suitable inert carrier gas such as hydrogen, nitrogen or argon and the heated grid or wire subjected to it for a period of time sufiicient to produce a coating of graphite on the exterior therof which is of substantial thickness.

As an xample of a specific process suitable for coating a tantalum Wire and providing thereon a graphite coating in the order of .001 to .005 inch thick, the following procedure has been employed with success. Suitable apparatus is illustrated in Fig. 3. A tantalum Wire 4 is partially enclosed in a suitable vessel 5 having an exhaust opening 6. The wire is heated as by passing current therethrough to a temperature in the range 1400 to 2000 C. The wire passes through the vessel and is supported by a pair of mercury cups provided with suitable glass inserts 8 which guide the Wire. Since the mercury 8a does not wet the glass it does not leak out along the wire. The heating current is supplied to the tantalum wire 4 through the mercury cups 7 from an alternating supply current 9 through suitable adjustable transforming means 10, the temperature being determined by the magnitude of the current supplied. For a relatively small wire, the temperature is preferably above 1500 C. since there is a certain heat transfer problem in heating the hydrocarbon vapor. The vessel within which the wire is enclosed communicates through conduit 11 with a flask 12 or other receptacle containing the hydrocarbon which, in a preferred example, is liquid benzene. The benzene is heated to a temperature in the order of 50 to C. to provide a substantially saturated benzene vapor and the benzene vapor is carried into the receptacle 5 enclosing the heated tantalum wire 4 by bubbling the carrier gas, specifically hydrogen, through the liquid benzene. The flask may be heated by a hot plate 13 and the hydrogen supplied by a conduit M which terminates in the flask below the level of the benzene. The hydrogen may be at a pressure of 1 atmosphere, for example, and bubble through the benzene at the rate of l to 5 liters per minute. The hydrogen acts merely as an inert atmosphere and carries the benzene vapor into the volume surrounding the heated tantalum wire. Upon contact with the hot wire the benzene decomposes and under the conditions specified forms a tightly-adhering layer of graphitic carbon. The graphitic carbon or graphite appears to be bonded to the tantalum Wire by an interface of a tantalum carbide. The thickness of the graphitic carbon layer may readily be controlled by controlling the time that the hot wire is ex posed to the vapor. For example, five to fifteen min utes with the other conditions as outlined above provide a coating approximately .001" while exposure from thirty minutes to one hour will provide a coating of approximately .005". It is apparent that for a continuous coating process the time of exposure is determined by the length of wire exposed and the speed of movement of the wire.

A Wire coated in the above manner exhibits superior properties not only from the standpoint of low grid emission at high temperatures but also from the standpoint of adherence of the graphitic carbon to the wire.

,, The graphitic carbon layer grades into a carbide layer which in turn grades into the core metal and the carbide interface mechanically bonds to both the core metal and the graphitic carbon outer layer.

columbium, molybdenum and tungsten may also be coated by the above process. Tantalum is preferred however because of the greater stability of the carbides of tantalum. It is apparent that the progressive formation of carbides of the core metal would eventually render the wire too brittle to be commercially valuable. The rapid rate of carbide formation resulting from contact of the hydrocarbon vapor with the hot wire limits the absorption of hydrogen by the core wire.

Other hydrocarbons than benzene may be employed as well as hydrocarbon derivatives as long as they produce a carbonizing vapor before decomposition. For example, carbon tetrachloride works very well. Since the decomposition of carbon tetrachloride is exothermic, the process may be carried out by heating the metal member or wire to be coated to a slightly lower temperature and temperatures as low as 1100 C. have been used successfully with carbon tetrachloride. When using carbon tetrachloride hydrogen should be used as the carrier gas since it enters into the chemical reaction. Such hydrocarbons as propane, methane and acetylene may also be employed.

In the foregoing description methods of preparing single wires have been described. Since the wires, after processing, are not readily bent without damaging the coating, it is also desirable to coat finished grid structures. In Fig. 2, I have shown a preformed wire grid structure which may be coated to provide a grid embodying my invention. As illustrated, the grid includes a pair of side rods with an over winding in the form of a flattened helix, preferably of tantalum wire. As shown in Fig. 4, this structure 16 may be placed in an enclosure in the form of a bell jar 17 and heated by a high frequency induction coil 18, for example. As illustrated, the grid is supported from the hooked end portion 19 of a glass rod 20. As illustrated, the jar 17 rests on a support 21 having an opening 22 through which the grids may be introduced and removed. This opening also provides a passage for the escape of gas supplied to the jar from the flask 23 which corresponds to the flask 12 of Fig. 3. There is some problem in coating grids of this character in that there is a difiiculty in heating all the parts to the same temperature. However, by proper attention to the heat transfer characteristics of the grid design and proper design of the high frequency induction coil or other heating device, reasonable uniformity of the coating may be achieved.

While I have given two specific examples of coating processes, it will be apparent that other processes which will provide the desired character of graphitic carbon on the surface bound to the core wire with a carbide of the core metal may also be employed.

It is an important advantage of the grids of the present invention that they are very rugged and durable as compared with prior art attempts to apply free carbon to the surface of a grid member. Grids manufactured in accordance with the invention have exhibited emissions in the order of /200 to that of molybdenum at temperatures of 1200 to 1600 C. in an electric discharge device employing thoriated-tungsten cathodes. These grid coatings have been found to be stable at temperatures of 1400 C. after 1000 hours of operation and stable for shorter periods at temperatures as high as 1600 C. It is readily seen that these coatings do provide a commercially practicable solution to grid emission problems, particularly in thoriated-tungsten-cathode tubes for use in the temperature range of 1200 to 1600 C., or above.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. A non-emissive electrode comprising a high-refractory metal core having an exterior coating of graphite and an intermediate bonding layer of a carbide of the core metal, said exterior coating comprising only substantially pure graphite at the outer surface thereof and graded into said bonding layer, and said bonding layer graded into said core metal.

2. A non-emissive electrode comprising a tantalum core, an exterior coating of graphite and an intermediate bonding layer of tantalum carbide, said exterior coating comprising only substantially pure graphite at the outer surface thereof and graded into said tantalum carbide, and said tantalum carbide graded into said tantalum.

References Cited in the file of this patent UNITED STATES PATENTS 1,852,865 Upp Apr. 5, 1932 1,865,449 Wuertz July 5, 1932 1,897,933 Guthrie Feb. 14, 1933 2,361,203 Holdaway et al. Oct. 24, 1944 2,400,893 Thurber et al. May 28, 1946 2,458,655 Sowa Jan. 11, 1949 2,497,111 Williams Feb. 14, 1950 2,658,844 Harbough Nov. 10, 1953 

1. A BON-EMMISIVE ELECTRODE COMPRISING A HIGH-REFRACTORY METAL CORE HAVING AN EXTERIOR COATING OF GRAPHITE AND AN INTERMEDIATE BONDING LAYER OF A CARBIDE OF THE CORE METAL, SAID EXTERIOR COATING COMPRISING ONLY SUBSUBSTANTIALLY PURE GRAPHITE AT THE OUTER SURFACE THEREOF AND GRADED INTO SAID BONDING LAYER, AND SAID BONDING LAYER GRADED INTO SAID CORE METAL. 